1. Field of the Invention:
This invention relates to a fluorescent composition, and more particularly to a fluorescent composition which can emit green, blue or red light under low velocity electron excitation under acceleration potential below 100 V.
2. Description of the Prior Arts:
A fluorescent display apparatus has been generally used in various electric or electronic display apparatus, because it can be driven with relatively low voltage, less power consumption, and exhibit bright and very clear displays. The fluorescent display apparatus performs the display of letters or patterns by impinging electrons emitted from filamentary cathodes when energized and heated upon anodes on which to deposit phosphor layers and to which to selectively apply anode voltage. The phosphor layers deposited on the anodes of the fluorescent display apparatus generally consists of a fluorescent composition which can give forth emission of high luminance by excitation of low velocity electrons, namely low acceleration voltage.
There is heretofore known a ZnO:Zn system fluorescent composition as a low velocity electron excited fluorescent composition which is used for the fluorescent display apparatus. The ZnO:Zn system fluorescent composition can be energized by luminance threshold voltage, that is, dead voltage of as low as 1 V to 2 V, and sufficient luminance for display can be obtained with anode voltage of 10 V to 20 V. Thus, it is extremely superior as the low velocity electron excited fluorescent composition. However, the ZnO:Zn system phosphor can only emit green light under electron excitation. Thus, the luminous color emitted from the fluorescent display apparatus using the ZnO:Zn system fluorescent composition is limited to green.
On the other hand, as the expansion of the applied field of the fluorescent display apparatus, there has been a great demand for diversification of the luminous color for display. For example, when effecting a warning signal, red is preferable for increasing the warning effect rather than green. Furthermore, when displaying several kinds of informations in one or plural fluorescent display apparatuses, each of the information can be correctly and extremely easy to recognize if the luminous color in each of the informations to be displayed is displayed in different colors. In order to meet such demand, various attempts have been made to develop a fluorescent composition which emits luminous color other than green by excitation of low speed electrons. For example, it has been proposed such low speed electron excited fluorescent compositions that are prepared by mixing various electric conductive materials with a ZnS:Ag or ZnS:Cu fluorescent composition used in the conventional cathode-ray tube which can emit various luminous color under high velocity electron excitation, or a (Zn, Cd)S:Ag or a Y.sub.2 O.sub.2 S:Eu fluorescent composition. Furthermore, it has also been proposed such a fluorescent composition that is prepared by mixing SnO.sub.2 which is originally an electric conductive material with rare earth elements, such as, for example, Eu.
In the fluorescent composition which has been produced heretofore by mixing the electric conductive material with the fluorescent composition, the emission of high luminance is not obtainable due to decrease of the luminance area in the fluorescent composition by admixture of the non-luminance electric conductive material in the fluorescent composition, and also the luminance is liable to be in irregular if the electric conductive material is not uniformly mixed therewith. The SnO.sub.2 :Eu fluorescent composition made of SnO.sub.2 to which Eu is added is not satisfactory in that it exhibits saturated characteristics under low luminance, and can not give forth emission of sufficient luminance. Therefore, it is unsatisfactory as a fluorescent composition for use in low velocity electron excitation. Thus, in the conventional fluorescent display apparatus with which a phosphor layer made of the fluorescent composition which is excited by low velocity electrons is provided in association with the ZnO:Zn phosphor layer in order to effect luminous display of different colors at the respective regions in one display portion, the driving circuit is complicated because of the difference in the luminance threshold voltage and the operating voltage of both phosphor layers. In addition each of the phosphor layers emits lights in different luminance, which makes the displays hard to observe. Furthermore, the operation life and the stability are not satisfactory.